How do you properly return multiple values from a Promise

Question

I ve recently run into a certain situation a couple of times  which I didn t know how to solve properly  Assume the following code  somethingAsync      then  afterSomething      then  afterSomethingElse      function afterSomething  amazingData       return processAsync  amazingData      function afterSomethingElse  processedData        Now a situation might arise where I would want to have access to amazingData in afterSomethingElse  One obvious solution would be to return an array or a hash from afterSomething  because  well  you can only return one value from a function  But I m wondering if there is a way to have afterSomethingElse accept 2 parameters and invoke it likewise  as that seems a lot easier to document and understand  I m only wondering about this possibility since there is Q spread  which does something similar to what I want

User · Answer

you can only pass one value  but it can be an array with multiples values within  as example   function step1      let server    myserver com     let data    so much data  very impresive     return Promise resolve  server  data        on the other side  you can use the destructuring expression for ES2015 to get the individual values   function step2  server  data      console log server      print  myserver com    console log data        print  so much data  very impresive    return Promise resolve  done        to call both promise  chaining them   step1    then step2   then  msg   gt     console log msg      print  done

User · Answer

Simply return a tuple      async add dto  TDto   Promise lt TDto gt        console log    this storeName  add   dto          return firebase firestore   collection this dtoName         withConverter lt TDto gt  this converter         add dto         then d   gt   d update this id  d id   d id  as  any  string          then x   gt  this get x 1

User · Answer

Two things you can do  return an object  somethingAsync        then  afterSomething        then  afterSomethingElse     function processAsync  amazingData           processSomething      return            amazingData  amazingData            processedData  processedData            function afterSomething  amazingData         return processAsync  amazingData       function afterSomethingElse  dataObj         let amazingData   dataObj amazingData          processedData   dataObj proccessedData      Use the scope   var amazingData  somethingAsync      then  afterSomething      then  afterSomethingElse    function afterSomething  returnedAmazingData       amazingData   returnedAmazingData    return processAsync  amazingData      function afterSomethingElse  processedData         use amazingData here

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Whatever you return from a promise will be wrapped into a promise to be unwrapped at the next  then   stage   It becomes interesting when you need to return one or more promise s  alongside one or more synchronous value s  such as   Promise resolve  Promise resolve 1   Promise resolve 2   3  4           then   p1 p2 n1 n2     gt     p1 and p2 are still promises        In these cases it would be essential to use Promise all   to get p1 and p2 promises unwrapped at the next  then   stage such as  Promise resolve Promise all  Promise resolve 1   Promise resolve 2   3  4            then   p1 p2 n1 n2     gt     p1 is 1  p2 is 2  n1 is 3 and n2 is 4

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Simply make an object and extract arguments from that object   let checkIfNumbersAddToTen   function  a  b    return new Promise function  resolve  reject     let c   parseInt a  parseInt b    let promiseResolution          c c       d   c c       x    RandomString       if c   10        resolve promiseResolution     else        reject  Not 10                Pull arguments from promiseResolution   checkIfNumbersAddToTen 5 5  then function  arguments    console log  c   arguments c   console log  d   arguments d   console log  x   arguments x     function  failure    console log failure

User · Answer

You can check Observable represented by Rxjs  lets you return more than one value

User · Answer

Here is how I reckon you should be doing   splitting the chain  Because both functions will be using amazingData  it makes sense to have them in a dedicated function  I usually do that everytime I want to reuse some data  so it is always present as a function arg   As your example is running some code  I will suppose it is all declared inside a function  I will call it toto    Then we will have another function which will run both afterSomething   and afterSomethingElse     function toto         return somethingAsync            then  tata        You will also notice I added a return statement as it is usually the way to go with Promises - you always return a promise so we can keep chaining if required  Here  somethingAsync   will produce amazingData and it will be available everywhere inside the new function   Now what this new function will look like typically depends on is processAsync   also asynchronous   processAsync not asynchronous  No reason to overcomplicate things if processAsync   is not asynchronous  Some old good sequential code would make it   function tata  amazingData         var processed   afterSomething  amazingData        return afterSomethingElse  amazingData  processed       function afterSomething  amazingData         return processAsync  amazingData      function afterSomethingElse  amazingData  processedData         Note that it does not matter if afterSomethingElse   is doing something async or not  If it does  a promise will be returned and the chain can continue  If it is not  then the result value will be returned  But because the function is called from a then    the value will be wrapped into a promise anyway  at least in raw Javascript    processAsync asynchronous  If processAsync   is asynchronous  the code will look slightly different  Here we consider afterSomething   and afterSomethingElse   are not going to be reused anywhere else   function tata  amazingData         return afterSomething            then  afterSomethingElse         function afterSomething     no args                return processAsync  amazingData              function afterSomethingElse  processedData                amazingData can be accessed here              Same as before for afterSomethingElse    It can be asynchronous or not  A promise will be returned  or a value wrapped into a resolved promise     Your coding style is quite close to what I use to do  that is why I answered even after 2 years  I am not a big fan of having anonymous functions everywhere  I find it hard to read  Even if it is quite common in the community   It is as we replaced the callback-hell by a promise-purgatory   I also like to keep the name of the functions in the then short  They will only be defined locally anyway  And most of the time they will call another function defined elsewhere - so reusable - to do the job  I even do that for functions with only 1 parameter  so I do not need to get the function in and out when I add remove a parameter to the function signature   Eating example  Here is an example   function goingThroughTheEatingProcess plenty  of  args  to  match  real  life        return iAmAsync            then chew           then swallow            function chew result                return carefullyChewThis plenty  of  args   water    piece of tooth   result                      function swallow wine                return nowIsTimeToSwallow match  real  life  wine                function iAmAsync         return Promise resolve  mooooore       function carefullyChewThis plenty  of  args  and  some  more        return true     function nowIsTimeToSwallow match  real  life  bobool        Do not focus too much on the Promise resolve    It is just a quick way to create a resolved promise  What I try to achieve by this is to have all the code I am running in a single location - just underneath the thens  All the others functions with a more descriptive name are reusable   The drawback with this technique is that it is defining a lot of functions  But it is a necessary pain I am afraid in order to avoid having anonymous functions all over the place  And what is the risk anyway  a stack overflow   joke      Using arrays or objects as defined in other answers would work too  This one in a way is the answer proposed by Kevin Reid   You can also use bind   or Promise all    Note that they will still require you to split your code   using bind  If you want to keep your functions reusable but do not really need to keep what is inside the then very short  you can use bind     function tata  amazingData         return afterSomething  amazingData            then  afterSomethingElse bind null  amazingData        function afterSomething  amazingData         return processAsync  amazingData      function afterSomethingElse  amazingData  processedData         To keep it simple  bind   will prepend the list of args  except the first one  to the function when it is called   using Promise all  In your post you mentionned the use of spread    I never used the framework you are using  but here is how you should be able to use it   Some believe Promise all   is the solution to all problems  so it deserves to be mentioned I guess   function tata  amazingData         return Promise all    amazingData  afterSomething  amazingData                then  afterSomethingElse       function afterSomething  amazingData         return processAsync  amazingData      function afterSomethingElse  args         var amazingData   args 0       var processedData   args 1       You can pass data to Promise all   - note the presence of the array - as long as promises  but make sure none of the promises fail otherwise it will stop processing   And instead of defining new variables from the args argument  you should be able to use spread   instead of then   for all sort of awesome work

User · Answer

You can return an object containing both values     there s nothing wrong with that   Another strategy is to keep the value  via closures  instead of passing it through   somethingAsync   then afterSomething    function afterSomething amazingData      return processAsync amazingData  then function  processedData           both amazingData and processedData are in scope here           Fully rather than partially inlined form  equivalent  arguably more consistent    somethingAsync   then function  amazingData      return processAsync amazingData  then function  processedData           both amazingData and processedData are in scope here

User · Answer

You can t resolve a promise with multiple properties just like you can t return multiple values from a function  A promise conceptually represents a value over time so while you can represent composite values you can t put multiple values in a promise   A promise inherently resolves with a single value - this is part of how Q works  how the Promises A  spec works and how the abstraction works   The closest you can get is use Q spread and return arrays or use ES6 destructuring if it s supported or you re willing to use a transpilation tool like BabelJS   As for passing context down a promise chain please refer to Bergi s excellent canonical on that

[javascript] How do you properly return multiple values from a Promise?

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